In the chemical, biomedical, bioscience and pharmaceutical industries, it has become increasingly desirable to perform large numbers of chemical operations in a highly parallel fashion. The formation of microfluidic concentration gradients can facilitate such operations. Microfluidic devices and systems provide improved methods of performing chemical, biochemical and biological analysis and synthesis. Chip-based microfluidic systems have been developed. Typically, such devices include chambers and reservoirs connected by channels. Reproducible and cost-effective devices, systems and methods for forming temporal and spatial microfluidic concentration gradients in 2D and 3D environments are desirable.